In the related art, there are phase-shifted full bridge type DC/DC converters. As shown in FIG. 1, a phase-shifted full bridge type DC/DC converter includes full-bridge type switching circuit 31 having four switching elements Sa, Sb, Sc and Sd. As shown in (a) to (d) of the time chart of FIG. 2, the phase-shifted full bridge type DC/DC converter outputs power according to a load by switching four switching elements Sa, Sb, Sc. and Sd.
In a fell-bridge type switching circuit, in time period Ton1 during which a pair of switching elements Sa and Sd are both turned on, input voltage Vi is output to a primary winding of transformer Tr, and a current flows to transformer Tr through switching elements Sa and Sd. Further, in time period Ton2 during which a pair of switching elements Sb and Sc are both turned on, input voltage Vi is output to the primary winding of transformer Tr in a reverse direction, and a reverse current flows to transformer Tr through switching elements Sb and Sc.
Four switching elements Sa, Sb, Sc, and Sd are controlled at a predetermined duty ratio. The duty ratio is a value obtained by adding or subtracting dead times Td1 and Td2 by 50%. When the load changes, by changing a switching phase between one and the other of the pair of switching elements Sa and Sd, time period Ton1 during which the current flows through switching elements Sa and Sd is changed. Similarly, by changing the switching phase between one and the other of the other pair of switching elements Sb and Sc, time period Ton2 during which the current flows through switching elements Sb and Sc is changed. In this way, in response to changes in the load, time periods Ton1 and Ton2 during which a current flows increase or decrease, and output power changes.
Further, in the phase-shifted full bridge type DC/DC converter, by controlling Zero Voltage Switching (ZVS), switching loss is reduced in the related art.
In the control of ZVS, between two switching elements Sa and Sb of which the input terminals are connected to each other in series and which are not turned on at the same time, delay is set from turning off one to turning on the other. This delay is dead tune Td1. Similarly, between the other set of two switching elements Sc and Sd that are not turned on at the same time, dead time Td2 is set from turning on one to turning on the other (see (a) to (d) of FIG. 2).
By setting such dead times Td1 and Td2, switching elements Sa, Sb, Sc, and Sd are turned on after both-end voltages Va, Vb, Vc, and Vd become zero volt respectively (see (e) to (h) of FIG. 2). Both-end voltages Va, Vb, Vc, and Vd are a source-drain voltage when switching elements Sa, Sb, Sc, and Sd are FETs.
After each corresponding both-end voltage Va, Vb, Vc, or Vd becomes zero volt, switching elements Sa, Sb, Sc, and Sd are turned on. In this way, in a time period during which ON-resistance is a value between zero and infinity, it is possible to suppress the flow of a current passing through each switching element Sa, Sb, Sc, or Sd. Accordingly, the power consumed in each switching element Sa, Sb, Sc, or Sd (switching loss) is reduced. Dead times Td1 and Td2 are normally set to one fourth of a resonance period determined from inductance avid capacitance values included in a circuit to be opened and closed by switching elements Sa, Sb, Sc, and Sd. The inductance and the capacitance values for generating resonance are, for example, resonance inductor L and parasitic capacitance Cr of switching elements Sa, Sb, Sc, and Sd.
Conventionally, in a phase-shifted full bridge type DC/DC converter for controlling ZVS, a technology to further improve the power conversion efficiency has been proposed (for example, see PTL 1).
In PTL 1, a saturable choke coil is provided in the rear stage of the four switching elements connected in a full bridge type, and by changing inductance of a circuit according to an amount of a load, unnecessary power loss is reduced. Further, in Embodiment 2 of PTL 1, a standard dead time changes in accordance with the change in the inductance of the saturable choke coil. Therefore, the ZVS is controlled by dynamically setting dead times in accordance with a standard dead time that changes.